The current supplied to cells of electrochemical plants, particularly to cells of metal electrowinning or electrorefining plants, may be distributed in a very diverse manner among the electrodes installed in said cells with negative consequences on production. Such phenomenon may occur for several reasons. For example, in the particular case of metal electrowinning or electrorefining plants, the electrodes of negative polarity (cathodes), are frequently removed from their seats to allow harvesting the product deposited thereon, to be later put back to their original locations for a subsequent production cycle. Such frequent handling, being generally carried out on a very large number of cathodes, often leads to an imperfect repositioning onto the respective current-collecting bus-bars giving rise to less-than-ideal electrical contacts, also due to possible fouling of the seats. Product deposition may additionally take place in an irregular fashion on the electrode surface, with formation of product mass gradients altering the surface profile of the cathode. Whenever this occurs, a state of electrical imbalance takes place caused by the anode-to-cathode gap being no longer constant along the whole surface: the electrical resistance, which is a function of the distance between each pair of anodes and cathodes, becomes variable, worsening the problem of irregular power distribution.
The current may therefore be apportioned in different extents to each electrode both due to bad electrical contacts between the electrode themselves and the current-collecting bus-bars and because of alterations of the surface profile of the cathodes. Moreover, even the simple wear of anodes may affect current distribution.
These inhomogeneities in the distribution of current can lead to anode-to-cathode short-circuits. In this case, the current tends to concentrate in the shorting areas with severe damage of the facing anodes. In addition, the short-circuit brings about a concentration of current on the affected cathode, diminishing current to the remaining cathodes and seriously hampering the production, which cannot be resumed until the shorted cathode is disconnected.
An uneven current distribution, besides generating a loss of quality and production capacity as mentioned, would also put at risk the integrity and lifetime of state-of-the-art anodes obtained starting from titanium meshes.
In industrial plants, given the high number of cells and electrodes present, the task of detecting irregularities in the distribution of current is very complex. Such measurement in fact involves thousands of manual measurements, carried out by operators via infrared or magnetic detectors. In the specific case of metal electrowinning or electrorefining installations, these detections are made by the operator in a high temperature environment and in the presence of acid mists, mainly consisting of sulphuric acid.
Moreover, conventional manual devices used by operators, such as gaussmeters or instruments with infrared sensors, allow to track down only large imbalances of current distribution, as they actually detect indirect imbalances generated by magnetic field or temperature variations, which in their turn are a function of local current intensity.
There are known systems for the wireless monitoring of cells which, in spite of being permanent and working in continuous, only detect changes in voltage and temperature for each cell and not for every single electrode. This information, as discussed above, is scarcely accurate and globally insufficient. In addition, there are now developmental projects aiming at the continuous detection of current supplied to individual cathodes by fixed current sensors relying on Hall effect: these sensors are active components that require a big size external power supply, such a large set of batteries.
Systems based on magnetic sensors are also known, nevertheless they do not offer a sufficient accuracy of measurement.
In conclusion, these manual or semi-manual systems have the disadvantage of not being suitable for continuous operation allowing only occasional checks; moreover, they have the disadvantage of being able to reveal only current variations of big magnitude in addition to being very expensive.
For these reasons, the industry is in need of a technically and economically viable system for permanently and continuously monitoring current distribution in all electrodes installed in cells of an electrowinning or electrorefining plant.